1. Field of the Invention
The present invention relates inductors and a method for manufacturing inductors. In particular, the present invention relates to an inductor and a method for manufacturing inductors, in which a molded magnetic body provided with a pair of external electrodes connected to an internal conductor-coil embedded in the molded magnetic body is made by molding a magnetic material which includes a powdered magnetic material and a resin.
2. Description of the Related Art
A conventional surface-mounting-type inductor 60 shown in FIG. 12 includes a coil (internal conductor-coil) 52 defining an inductance element is embedded in a molded magnetic body 53 formed by molding a magnetic material 51 including a powdered magnetic material and a resin. The molded magnetic body 53 is provided at ends thereof with a pair of external electrodes 54a and 54b connected to the coil 52 at ends 52a and 52b, respectively, of the coil 52.
The inductor 60 is manufactured, for example, such that a coil (an air-core coil) formed by densely winding an insulative covered copper wire and cutting the same by a predetermined length is provided in a mold, a magnetic molding compound made by kneading a powdered magnetic material and a resin is injected into the mold and is provided around the coil (inside and outside the coil), and the mold is released, thereby producing a molded magnetic body. The molded magnetic body is provided with external electrodes made of metallic films at ends of the molded magnetic body including exposed portions of the coil, the external electrodes are formed by coating, baking, deposition, or sputtering of a conductive paste, such that the external electrodes are connected to the exposed portions of the coil.
The inductor 60 can be manufactured only by forming the molded magnetic body 53 by molding the magnetic material 51 which is made by kneading a powdered magnetic material and a resin, and providing the external electrodes 54a and 54b made of metallic films. Therefore, a firing process at a high temperature and a baking process for the electrodes, which are necessary in manufacturing a conventional ceramic inductor including a magnetic ceramic, are not necessary, whereby manufacturing costs are reduced.
In the inductor 60, the external electrodes 54a and 54b are arranged to connect to exposed portions 52a and 52b which are portions of final windings of wire of a coil 52. The shape and the position (for example, the position in a vertical direction of the exposed portion 52a or 52b) of the exposed portion 52a or 52b of the coil 52 often differ according to each inductor 60 due to deformation of the coil 52 during injection of the magnetic material 51.
In a conventional manufacturing method, since the coil 52 is deformed due to being pressed by the mold when the length of the coil 52 is greater than that of the molded magnetic body 53, the length of the coil 52 must be substantially the same as that of the molded magnetic body 53. Therefore, as shown in FIG. 13, the exposed portion 52a or 52b of the coil 52 is formed partially in the final winding of wire of the coil 52 at the end of the molded magnetic body 53, and the area of the exposed portion 52a or 52b is reduced because of the difficulty in forming the exposed portion 52a or 52b which significantly protrudes from the end of the molded magnetic body 53.
Therefore, the connection between the coil 52 and the external electrodes 54a and 54b is not secure and an overcurrent is applied to the coil.
In another conventional inductor, the inductor 60 is provided with the external electrodes 54a and 54b which are defined by a plurality of layers such that the external electrodes 54a and 54b are easily soldered, a metallic film, such as solder, tin, or silver, to which solder easily adheres, being used as an outermost layer. When the inductor 60 is mounted on a mounting body such as a printed circuit board 61 via a method such as reflow-soldering, as shown in FIG. 14, a solder fillet 62 is raised to a height Hs which is at least ⅓ of a height H of the inductor 60 because the solder easily adheres to the external electrodes 54a and 54b. The inductor 60 is mounted such that the solder fillet 62 is electrically connected to the external electrodes 54a and 54b. 
In the conventional method of manufacturing an inductor, a magnetic molding compound is injected into the mold in which a coil is not firmly affixed in a desired position in the mold. Therefore, there is a risk that the coil will move depending on the direction of flow of the magnetic molding compound during the injection process.
For example, when the inductor 60 in which the coil 52 is displaced, as shown in FIG. 15, is mounted on the printed circuit board 61, the solder fillet 62 does not reach the positions of the exposed portions 52a and 52b of the coil 52 with the external electrodes 54a and 54b therebetween even when the solder fillet 62 is raised to the height Hs which is at least ⅓ of the height H of the inductor 60, because the exposed portions 52a and 52b of the coil 52 are excessively elevated, and a gap G is produced between a lower end of the exposed portion 52a or 52b and an upper end of the solder fillet 62. The current applied to the inductor 60 flows through only the external electrodes 54a and 54b at the gap portion. Therefore, when the external electrodes 54a and 54b are made of a metallic thin film such as a solder film, long-term reliability and unsafe operation when an overcurrent is applied occur, due to insufficient current capacity in the portion having the gap.
To overcome these problems, the thickness of the metallic film defining the external electrodes 54a and 54b may be increased. However, the manufacturing costs also increase with the increased thickness of the film.
The external electrodes 54a and 54b may be formed by bonding metallic plates to the ends of the molded magnetic body 53, each of the metallic plates having a sufficient thickness required for the current capacity. However, the manufacturing costs are also increased with this method.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide an inductor and a method for manufacturing the inductor, in which reliable connection between an internal conductor-coil and external electrodes, long-term reliability after mounted, and safety when applied with an overcurrent are achieved.
An inductor according to a preferred embodiment of the present invention includes a molded magnetic body formed by molding a magnetic material including a powdered magnetic material and a resin-based material, an internal conductor-coil embedded in the molded magnetic body such that both ends of the internal conductor-coil are exposed from both end surfaces of the molded magnetic body, respectively, and a pair of external electrodes provided at the respective end surfaces of the molded magnetic body to connect to the internal conductor-coil at the respective ends thereof. At least two thirds of a final winding of wire at each of the ends of the internal conductor-coil project from the end surface of the molded magnetic body by at least about one fifth of the diameter of the wire of the internal conductor-coil. The external electrodes are each connected with at least about two thirds of the final winding of wire at each of the ends of the internal conductor-coil, which project from the end surface of the molded magnetic body by at least about one fifth of the diameter of the wire of the internal conductor-coil.
At least about ⅔ of a final winding of wire at each end of the internal conductor-coil project from the end surface of the molded magnetic body by an amount of at least about ⅕ of the diameter of a wire, and the external electrodes are each connected with at least about ⅔ of the final winding of wire at each of the ends of the internal conductor-coil, which project from the end surface of the molded magnetic body by at least about ⅕ of the diameter of the wire of the internal conductor-coil, whereby reliable connection is established by increasing the area of connection between the internal conductor-coil and the external electrodes, and long-term reliability after mounted and safety when applied with an overcurrent are greatly improved. Moreover, the thickness of the external electrodes is greatly reduced, thereby greatly reducing the manufacturing costs.
The resin-based material used together with the powdered magnetic material, according to various preferred embodiments of the present invention, includes various materials, such as an epoxy resin, a synthetic resin including polyphenylene sulfide, and a rubber resin including a chloroprene rubber or a silicone rubber.
The external electrodes are preferably defined by a plurality of layers of metallic films.
When each external electrode is defined by a plurality of layers, an inductor having reliable electrical connection and solderability is provided by depositing a tin-plating film or a solder-plating film on a base metallic film defining the external electrodes.
The center of the final winding of wire at each of the ends of the internal conductor-coil is spaced away from the center of each end surface of the molded magnetic body by a distance not greater than about half of the inner diameter of the internal conductor-coil.
Since the center of the final winding of wire at each of the ends of the internal conductor-coil is spaced away from the center of each end surface of the molded magnetic body by a distance not greater than about xc2xd of the inner diameter of the internal conductor-coil, the condition described below is efficiently prevented from occurring. That is, when an inductor in which an internal conductor-coil is displaced is mounted on a printed circuit board, a solder fillet does not reach a position where the solder fillet is opposed to an exposed portion of the coil with external electrodes therebetween because the position of the exposed portion of the internal conductor-coil is excessively elevated, and a gap is produced between a lower end of the exposed portion and an upper end of the solder fillet. Therefore, when the external electrodes are made of a metallic thin film such as a plating film, reliability and safety are substantially diminished when an overcurrent is applied, due to insufficient current capacity in the portion corresponding to the gap. These problems are prevented by preferred embodiments of the present invention.
According to another preferred embodiment of the present invention, a method for manufacturing an inductor includes the steps of preparing the internal conductor-coil, setting the internal conductor-coil in a mold, coupling the internal conductor-coil with a coil-supporting member at an inner periphery of the internal conductor-coil for supporting the internal conductor-coil at the inner periphery thereof, thereby preventing the internal conductor-coil from being deformed and maintaining the internal conductor-coil in a position and shape in which the internal conductor-coil is disposed to be exposed from a magnetic material at ends of the internal conductor-coil, and a first injection step of injecting the magnetic material through a gate provided at a predetermined position of the mold into a region of the mold except for a region at the inner periphery of the internal conductor-coil in which the coil-supporting member is disposed, removing the coil-supporting member after the magnetic material injected in the first injection step cures, and a second injection step of injecting the magnetic material into the region at the inner periphery of the internal conductor-coil through another gate provided at a predetermined position of the mold, thereby forming a molded magnetic body in which a major portion of the internal conductor-coil is embedded in the molded magnetic body and at least about two thirds of a final winding of wire at each end of the internal conductor-coil project from an end surface of the molded magnetic body by at least about one fifth of the diameter of the wire of the internal conductor-coil, and forming a pair of external electrodes at the respective end surfaces of the molded magnetic body so that the external electrodes are each connected with at least about two thirds of the final winding of wire at each of the ends of the internal conductor-coil, which project from the end surface of the molded magnetic body by at least about one fifth of the diameter of the wire of the internal conductor-coil.
The internal conductor-coil is supported by the coil-supporting member at the inner periphery of the internal conductor-coil so as to prevent the internal conductor-coil from being deformed and to maintain the internal conductor-coil in a position and a shape in which the internal conductor-coil is disposed so as to be exposed from a magnetic material at ends of the internal conductor-coil, the magnetic material is injected into a region of the mold except for a region at the inner periphery of the internal conductor-coil, the coil-supporting member is removed after the magnetic material cures, and the magnetic material is injected into the region at the inner periphery of the internal conductor-coil, thereby forming a molded magnetic body in which at least about ⅔ of a final winding of wire at each end of the internal conductor-coil project from an end face of the molded magnetic body by at least about ⅕ of the diameter of a wire of the internal conductor-coil. A pair of external electrodes are provided at the respective end surfaces of the molded magnetic body such that the external electrodes are each connected with at least about ⅔ of the final winding of wire at each of the ends of the internal conductor-coil, which project from the end surface of the molded magnetic body by at least about ⅕ of the diameter of the wire of the internal conductor-coil. Thus, the inductor according to preferred embodiments of the present invention is efficiently and reliably manufactured.
The mold is provided with substantially annular concave portions, each of the annular concave portions is provided at an inner surface of the mold opposing the end of the internal conductor-coil such that at least one portion of the final winding of wire at the end of the internal conductor-coil is fitted with the annular concave portion.
By using the mold which is provided with substantially annular concave portions, each of the annular concave portions at an inner surface of the mold opposing the end of the internal conductor-coil and at least one portion of the final winding of wire at the end of the internal conductor-coil is fitted with the annular concave portion, a molded magnetic body, in which at least about ⅔ of a final winding of wire at each end of the internal conductor-coil project from an end surface of the molded magnetic body by at least about ⅕ of the diameter of the wire of the internal conductor-coil, is reliably produced.
The center of each substantially annular concave portion provided at the inner surface of the mold and the center of each end surface of the molded magnetic body substantially correspond to each other.
When the centers of each substantially annular concave portion provided at the inner surface of the mold and each end surface of the molded magnetic body substantially coincide with each other, a risk of a phenomena described below is efficiently avoided. That is, when an inductor in which an internal conductor-coil is displaced is mounted on a printed circuit board, a solder fillet does not reach a position where the solder fillet is opposed to an exposed portion of the coil with external electrodes therebetween because the position of the exposed portion of the internal conductor-coil is excessively elevated, and a gap is produced between a lower end of the exposed portion and an upper end of the solder fillet. Therefore, when the external electrodes are made of a metallic thin film such as a plating film, long-term reliability and safety is substantially diminished when an overcurrent is applied, due to insufficient current capacity in the portion corresponding to the gap. These problems are prevented by the preferred embodiments of the present invention.
Other features, elements, characteristics and advantages of the present invention will become apparent from the detailed description of preferred embodiments thereof with reference to the attached drawings.